World Statistics and Facts: Age of the Earth.

Age of the Earth

The age of the Earth is estimated to be 4.55 billion (4.55 × 109) years, based on detailed scientific evidence. This estimate represents a compromise between the oldest known terrestrial minerals – small crystals of zircon from the Jack Hills of Western Australia – and astronomers' and planetologists' estimates of the age of the solar system. The radiometric age dating evidence from the zircons further confirms that the Earth is at least 4.404 billion years old. Comparing the mass and luminosity of the Sun to the multitudes of other stars, it appears that the solar system can not be much older than those rocks. Ca-Al-rich inclusions – the oldest known solid constituents within meteorites which are formed within the solar system – are 4.567 billion years old, giving an age for the solar system and an upper limit for the age of the Earth. It is assumed that the accretion of the Earth began soon after the formation of the Ca-Al-rich inclusions and the meteorites. Since the accretion time of the Earth is not exactly known yet and the predictions from different accretion models vary between several millions up to about one hundred million years, the exact age of the Earth is difficult to define.

In the centuries preceding the scientific revolution, the age of the Earth was determined from the accounts of creation by religious authority. The Han Chinese thought the Earth was created and destroyed in cycles of over 23 million years. Westerners were more conservative. In a book published in 1654, not long before his death, Archbishop James Ussher of Armagh, Ireland, calculated from the Bible (augmented by some astronomy and numerology) that creation began on October 23, 4004 BC.

Few people had conceived the idea of deep time that stretched far into the past before the arrival of humankind, or far into the future beyond the end of humankind. One who did was Aristotle, who thought the Earth and universe had existed from eternity.

By the 18th century, a few naturalists were trying to place the age of the Earth on a more scientific basis. The naturalist Mikhail Lomonosov, regarded as the founder of Russian science, was one of the first to undertake this exercise, suggesting in the mid-18th Century that the Earth had been created separately from the rest of the universe, several hundred thousands of years before.

Lomonosov's ideas were mostly speculative, but in 1779 the French naturalist the Comte du Buffon tried to obtain a value for the age of the Earth using an experiment. He created a small globe that resembled the Earth in composition and then measured its rate of cooling. This led him to estimate that the Earth was about 75,000 years old.

In geology, rock samples often contained fossilized remains of unknown creatures, and there seemed to be a progression of types of such creatures from layer to layer. In the 1790s, the British naturalist William Smith pointed out that if two layers of rock at widely differing locations contained similar fossils, then it was very plausible that the layers were the same age.

In 1830, the geologist Charles Lyell took the next step and proposed that the features of the Earth were in perpetual change, eroding and reforming continuously, and the rate of this change was roughly constant.

In 1862, the physicist William Thomson of Glasgow published calculations that fixed the age of the Earth at between 20 million and 400 million years. He assumed that the Earth had been created as a completely molten ball of rock, and determined the amount of time it took for the ball to cool to its present temperature.

Geologists quickly realized that the discovery of radioactivity upset the assumptions on which most calculations of the age of the Earth were based. These calculations assumed that the Earth and Sun had been created at some time in the past and had been steadily cooling since that time. Radioactivity provided a process that generated energy. George Darwin and Joly were the first to point this out, also in 1903.

There was the issue of whether the Earth contained enough radioactive material to significantly affect its rate of cooling. In 1901 two German schoolteachers, Julius Elster and Hans F. Geitel, had detected radioactivity in the air and then in the soil. Other investigators found it in rainwater, snow, and groundwater. Robert J. Strutt of Imperial College, London, found traces of radium in many rock samples, and concluded that the Earth contained more than enough radioactive material to keep it warm for a long, long time.

Radiometric dating continues to be the predominant way scientists date geologic timescales. Techniques for radioactive dating have been tested and fine tuned for the past 50+ years. Forty or so different dating techniques are utilized to date a wide variety of materials, and dates for the same sample using these techniques are in very close agreement on the age of the material. Possible contamination problems do exist, but they have been studied and dealt with by careful investigation; leading to sample preparation procedures being minimized to limit the chance of contamination. Hundreds to thousands of measurements are done daily with excellent precision and accurate results. Even so, research continues to refine and improve radiometric dating to this day.

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